[Bug target/108742] Incorrect constant folding with (or exposed by) -fexcess-precision=standard

["matz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108742

--- Comment #10 from Michael Matz <matz at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #9)
> (In reply to Michael Matz from comment #7)
> > (In reply to Jakub Jelinek from comment #5)
> > > https://eel.is/c++draft/cfloat.syn points to the C standard for
> > > FLT_EVAL_METHOD
> > > (plus https://eel.is/c++draft/expr#pre-6 talks about excess precision too)
> > > and e.g. C17
> > > 5.2.4.2.2/9):
> > > "2 evaluate all operations and constants to the range and precision of the
> > > long double type."
> > > 
> > > Note the " and constants" above.
> > 
> > Yes.  But that leaves unspecified exactly to what bit pattern the string
> > "4.2" should be converted to.
> 
> It should be converted to the closest long double, which is
> 0x8.6666666666668p-1,
> otherwise the constants wouldn't be evaluated to the range and precision of
> the long double type, only to double type then extended to long double.

Yes.

> In that case there
> would be no point to mention the " and constants" above, only operations
> would have excess precision, so double d; ... d = d + 4.2; would be d =
> (double) ((long double) d + (long double) (double) 4.2), while it actuall
> should be d = (double) ((long double) d + 4.2L);

As is clear by now, I disagree on that.  FLT_EVAL_METHOD is for dealing with
excess presicion hardware in a predictable way.  x87 loads double constants
into the 80bit regs trivially (with the value I want "(long double)4.2" to
have, not
4.2L), so that's what the frontend should do.  I think an argument that
involves that the standard otherwise "would have no point mentioning" something
is
slippery at best.

> Sure, see the typeof above, 4.2 with FLT_EVAL_METHOD == 2 has typeof double,
> but value of 4.2L.

I think we'll have to agree to disagree here and let Joseph have the say :-)